Many water amateurs and companies that want to promote water filters for tap water use TDS (Total Dissolved Solids) as the key water quality testing method. The reason is that it’s very easy and cheap to measure. The problem is that this is not a good measure and it’s very limited in terms of water quality parameters.
TDS refers to total dissolved solids in the water. Dissolved solids refer to any minerals, salts, metals, cations or anions dissolved in water. Total dissolved solids (TDS) comprise inorganic salts (principally calcium, magnesium, potassium, sodium, bicarbonates, chlorides, and sulfates) and some small amounts of organic matter that are dissolved in water.
Typically, natural mineral water and tap water can have a TDS value of 100-200 mg/l. In areas with high concentration of minerals, the natural tap water could however be considerably higher. Water filtering technologies such as reverse osmosis, water distillation and ion exchange can reduce this to close to zero, whereas active carbon filters will not filter out TDS
So is a higher level of TDS good or bad?
Just like most bottled mineral water contains higher TDS (e.g. Evian 300 mg/l; San Pellegrino 850 mg/l), this can also be positive for tap water. WHO and most other institutions that regulate water quality consider values up to 600 mg/l to be entirely safe and 2,000 mg/l safe for temporary consumption in case no other water is available.
For values below 600 mg/l there is no scientific evidence that TDS makes any difference, and no health impact has been identified below 2,000 mg/l. To complicate this a little bit there is however evidence that high concentrations of specific constituents such as calcium may have an impact, but TDS is a very vague indicator of this, since it does not specify the different parameters that constitute the final number.
In a study by the World Health Organization, a panel of tasters came to the following conclusions about the preferable level of TDS in water (mg/l):
- 50 – 300: Excellent*
- 300 – 600: Good
- 600 – 900: Fair
- 900 – 1,200: Poor
- Above 1,200: Unacceptable
* The testers in the report by WHO noted that water with less than 50 might taste flat but this is aesthetic rather than a health concern.
Why do water filter companies and amateurs use high TDS as an argument for water filtration?
The simple reason is that it’s cheap and easy to test. A TDS meter can be purchased online for very little money, and then anyone can easily perform a test in seconds at home. Some filtration companies even include TDS meters with their filter systems and pitchers because of the low cost and because it gives their consumers some peace of mind.
Real tests that measure chlorine, pesticides, heavy metals and pharmaceuticals, which are important to determine water quality, require sophisticated and expensive labs. Your local water company tests the water every day for contaminants and is obliged to provide a water quality report, at least, yearly. This provides a scientific analysis of the water quality. Water quality test kits are readily available and can be found by clicking here.
TDS meters that measure the electrical charge in the water and is not an effective tool for measuring the effectiveness of a refrigerator filter. Refrigerator filters use a granular activated carbon block for the removal of chlorine and other organic molecules and do not affect TDS in the water.